Grinding surfaces for treating aqueous suspension

Abstract

A grinding element (2) for a refiner, comprising grinding strips (21) and grooves (22) running between the grinding strips (21), wherein some of the grinding strips (21) split radially outwards in a splitting region (26) and the grinding strips (21) have a smaller thickness in the circumferential direction in at least one portion after the split (27) in the splitting region (26) than outside the splitting region (26).

Description

[0001] Grinding surfaces for the treatment of aqueous suspension

[0002] The invention relates to grinding plates with grinding surfaces for grinding an aqueous suspension, preferably for grinding suspended cellulose fibers, between two grinding surfaces forming a grinding gap and rotating relative to each other. The grinding surfaces are formed by grinding ribs and grooves running between them.

[0003] It has long been known to grind cellulose fibers, i.e., virgin pulp and / or recycled paper fibers, in order to achieve the desired properties in the resulting fiber web, especially with regard to strength, formation and surface.

[0004] In the refiners used, the grinding surfaces are formed by replaceable grinding plates, also known as grinding assemblies, which are screwed to the corresponding support surface, due to the relatively rapid wear.

[0005] To achieve the desired fiber properties, especially the degree of grinding, the grinding sets must be adapted as closely as possible to the fiber material being treated, also to prevent excessive wear of the sets.

[0006] EP 2 722 433 shows grinding plates or grinding plate segments with grooves and ribs. The grooves of the grinding plate segments are either essentially completely blocked by full-height dams or partially blocked by full-height dams. In some designs, the dams are only located in the radially outer area between the grinding ribs.

[0007] From EP 1 670 592 B1, grinding plate segments with grinding guides are known. The grinding guides are radially oriented. This allows these grinding plate segments to be used independently of the direction of rotation, and the direction of rotation can be changed during operation. EP 4063561 and EP 3450624 show grinding plates with dams. The dams are arranged at an angle relative to the guides to allow the return of any steam that forms to the radial center. Recesses are provided for the steam to pass through, so that the steam can take its path radially inward within the grinding surface and does not enter the gap formed between the grinding surfaces.

[0008] The invention is based on the objective of improving the efficiency of fiber treatment.

[0009] The problem is solved according to the invention by an embodiment according to the independent claim. Further advantageous embodiments of the present invention are found in the dependent claims.

[0010] By ensuring that the grinding bars of the grinding element have a reduced circumferential thickness in at least one section within the divided area compared to the area outside the divided section, the fibers can be treated by the grinding elements with this reduced thickness. This increases the efficiency of such grinding elements.

[0011] In a preferred embodiment, the grinding bars are at least 10% thinner after division. It is particularly preferred that the grinding bars are at least 20% thinner, up to a maximum of 30% thinner, after division. A thickness of 25% thinner immediately after division is especially preferred. This ensures that the grinding bars are sufficiently stable for the stresses encountered, and the thinner grinding bars can then be used for additional processing of the fiber suspension.

[0012] In a preferred embodiment, the grinding elements are designed with openings for fastening in the grinding assembly. The grinding elements are subject to wear and must be replaced regularly. Openings for fastening are frequently provided in the grinding elements. However, it has been shown that higher loads can occur on the grinding bars adjacent to the openings. To provide particularly robust grinding elements, it has proven advantageous to thicken the grinding bars adjacent to the openings in the circumferential direction. The thickened grinding bars have a greater thickness in the circumferential direction. This allows the grinding bars in this area to withstand a higher load.

[0013] In a preferred embodiment, the thickened or reinforced grinding bars are part of a division section prior to the division. This allows for optimal utilization of the grinding surface geometry of the grinding elements. The reinforced sections of the grinding bars can then be further divided. This arrangement contributes to a dense arrangement of grinding bars, making such grinding elements particularly efficient.

[0014] In one embodiment, dams are formed between circumferentially adjacent grinding bars. Suspension flowing in the grooves is conveyed by these dams into the grinding gap. Thus, the dams contribute to improved suspension treatment. The dams are particularly preferably located in the radially outer region, so that improved mixing of the suspension from the grooves with the suspension flowing in the gap is achieved, promoting more homogeneous suspension treatment.

[0015] In particular, the grinding elements are designed for LC grinding:

[0016] LC milling refers to the treatment of suspensions in the range of 3% to 6% by weight.

[0017] It has proven particularly advantageous to position the grinding elements with dams within the stator of a grinding arrangement. The dams allow backflow to be controlled and / or reduced. This has a particularly positive effect on power loss / idle power consumption. In a preferred embodiment, the grinding bars are arranged radially outside the indexing section. This allows these grinding elements to be used independently of rotation. If the grinding elements are cast, the number of required molds is reduced, which has a beneficial effect on manufacturing costs. Furthermore, simplified inventory management is possible due to the reduced number of variants.

[0018] In a preferred embodiment, the grinding bars in the section after the division have a reduced thickness in the circumferential direction over at least 1 / 3, preferably 2 / 3, of the length of the section after the division. The groove between the tapered grinding bars can first be brought to the groove width outside the division area, which has an advantageous effect on the flow behavior.

[0019] In a preferred embodiment, it is provided that the width of the grooves in the circumferential direction is the same outside the division sections.

[0020] The grooves are machined with the minimum possible width. Making them narrower would increase the risk of clogging. Conversely, making the grooves wider than necessary would reduce the edge length and thus impair the processing capacity. The goal is to maximize the edge length while ensuring stable operation.

[0021] In a preferred embodiment, it is provided that the grinding bars are of the same thickness outside the division sections.

[0022] In a preferred embodiment, dams are provided in the grooves, preferably in the radially outer region extending over two-thirds of the radial extent of the grinding element. This allows for improved fiber treatment. The dams optimize the hydraulic capacity. It has been shown that with reduced height of treatment elements, the flow direction in the stator can reverse from backflow to forward flow. Providing dams in the grooves prevents this forward flow.

[0023] On the other hand, a targeted increase in hydraulic capacity may be desired in the radially inner area in order to supply as much suspension as possible to the grinding arrangement.

[0024] The hydraulic capacity of the grinding surfaces typically decreases with increasing wear, and premature replacement of the grinding elements may be necessary due to insufficient hydraulic capacity. By significantly increasing the grinding bar height in the radially inner area, the open area in the inner region has been enlarged. This increases the hydraulic capacity and enables a longer service life.

[0025] The grinding bars and the dams are the treatment elements. A reduction in height occurs due to wear.

[0026] Adjacent grinding bars are particularly favored when connected by the dams. This contributes to the stability of the grinding surface.

[0027] In a preferred embodiment, an obtuse angle is provided radially inward between the dam and the grinding bar at the radially inner connection point. This allows the dams to function as grinding bars and also to treat the fibers of the fiber suspension. This further contributes to the efficiency of the grinding elements.

[0028] Grinding arrangement for grinding aqueous-suspended cellulose fibers between two grinding surfaces forming a grinding gap and rotating relative to each other. At least one of the grinding surfaces has at least one previously described grinding element.

[0029] In a grinding arrangement, at least one grinding surface is preferably provided with dams. The dams are connected at an obtuse angle radially inside a grinding bar to form a treatment edge. The suspension flows towards the dam on the side of the radially inside obtuse angle of the dam and is treated by this boundary edge of the dam. This oblique orientation of the dams increases the efficiency of the grinding arrangement, as the dams also act as treatment elements in addition to the grinding bars.

[0030] In a preferred embodiment, the grinding bars (21) are designed with an axial extent that is at least 15%, preferably 20%, greater radially inward than radially outward. Preferably, the variation in the axial extent of the grinding bars is not greater than 25% from the axial extent of the grinding bars in the radially outer region. The increase in the height of the grinding bars is gradual. The height of the grinding bars starting radially inward is considered to be the height after reaching the maximum height, i.e., after the end of the radially inward chamfer. The end of the grinding bars is considered radially outward up to -5% radially inward. By varying the height of the grinding bars, the capacity of the grinding surface in the inner region can be increased. This has a positive effect on the usability of the grinding assembly even in the case of wear.

[0031] The flow direction of the suspension is determined by the relative movement of the grinding surfaces and, in the case of radial inward feeding, always has a flow component towards the radial outward.

[0032] The invention will be explained below with the aid of figures. The figures show, in detail:

[0033] Fig. 1: Schematic cross-section through a grinding arrangement

[0034] Fig. 2: Segment of a grinding plate with dams in the radially outer area

[0035] Fig. 3: Representation of a section of a segment of a grinding plate

[0036] Fig. 4: Detailed view of the branching area

[0037] Fig. 5: Grinding plate segment with recesses for fastening

[0038] Fig. 6: Grinding bars

[0039] Fig. 7: Superimposed view of the grinding surfaces. Figure 1 shows a grinding arrangement 1. In the grinding arrangement 1, a grinding gap 3 is formed by a stationary grinding surface coupled to the housing and a grinding surface 4 rotating about a rotation axis 10. Such grinding arrangements for treating fiber-containing suspensions are also called refiners. Refiners are used in particular in the fiber processing of cellulose fibers.

[0040] The two annular grinding surfaces 4 run parallel to each other, with the distance between them typically being adjustable. In addition to the flat grinding surfaces 4 shown here, conical grinding surfaces, also known as treatment surfaces, are possible. The rotating grinding surface 4 is moved in the direction of rotation by a shaft 12, which is rotatably mounted in the housing. This shaft 10 is driven by a drive mechanism (not shown).

[0041] In the example shown, the fiber suspension 1 to be ground enters the grinding gap 3 between the two grinding surfaces 4 via an inlet through the center. However, feeding via openings in the grinding surface is also possible. The fiber suspension S passes radially outwards through the interacting grinding surfaces 4 and exits the subsequent annular space through an outlet 6.

[0042] Not shown are the means, known per se, by which a force is generated to press the two grinding surfaces 4 against each other. Each grinding surface 4 is formed by several circular segment or annular segment-shaped grinding segments 8 as grinding elements according to Figure 2. However, the grinding surface 4 could also be formed by a single grinding element 2. The grinding elements 2 have a base plate 18. Treatment elements 20, here grinding bars 21 and dams 23, are provided on the base plate. In the illustration according to Figure 2, the dams formed between the grinding bars 21 are located in the radially outer region.

[0043] The grinding segments 8 extend circumferentially and are arranged side by side circumferentially. Each grinding segment 8 is formed by a base plate 18 with a plurality of treatment elements 20 and intervening grooves 22. The treatment elements 20 are grinding bars 21 and dams 23.

[0044] Parallel to the base plate 18, the grinding bars 21 have an elongated cross-sectional shape, with the upper side of the grinding bars 21, which faces the treatment gap 3, generally running parallel to the outer surface of the base plate 18.

[0045] The following section describes the dividing section 26 in more detail with reference to Figure 3. Radially inward from the division 27, the dividing section 26 has a thickened grinding bar in the region 28. After the division into two grinding bars 21, the grinding bars 21 are tapered over a partial region 29 of the section following the division 27. Outside the dividing section 26, the grinding bars 21 have identical thickness. The grooves 22 have constant diameters outside the dividing sections. In the radially outer region, here approximately on the outer 2 / 3 of the grinding surface, dams 23 are provided in the grooves 22. The dams 23 are connected to the adjacent grinding bars 21. The dams 23 are arranged at an angle to the grinding bars 21 and have an obtuse angle 24 radially inward. The orientation of the dams 23 depends on the flow direction of the suspension in the grinding gap 3.The radially inward-facing boundary edge of the dams 23 is exposed to the flow of the suspension. This boundary edge of the dams 23 thus acts as a treatment edge 25 and contributes to the treatment efficiency. Due to the intended inclined orientation of the dams 23 to provide a treatment edge 25, the grinding elements are preferably used in accordance with the direction of rotation. In a double-disc refiner, the grinding surfaces with grinding elements with differently oriented dams 23 are to be used if the grinding surfaces of the rotor 16 are arranged axially between the grinding surfaces of the stator.

[0046] If no dams 23 are provided and the grinding bars run radially + / - 5°, the grinding elements 2 can be used regardless of the direction of rotation while maintaining the same efficiency. Figure 4 shows an enlarged section of the division. The groove 22 after the division

[0047] The section 27 of a grinding bar 21 is extended radially inwards by a region 31, in particular to increase the edge length of the grinding bars 21. The thickness 30 of the grinding bar 21 is reduced at the same time; however, due to the connection of the grinding bars 21 before the division 27, the grinding bars 21 are mechanically reinforced in this region, and the reduced thickness of the grinding bars in region 29 does not result in any loss of load-bearing capacity.

[0048] Figure 5 shows a grinding segment with openings 35 for mounting in a grinding arrangement 1. Grinding bars with thickened areas 28 are arranged circumferentially adjacent to the openings 35. Here, the thickened areas

[0049] 28 of the grinding bars 21 part of a division section 26.

[0050] Optimized edge length:

[0051] The provision of dams 23 has the disadvantage that the cutting edge length of the grinding bars 21 is reduced. To compensate for this loss and even to increase the cutting edge length of the grinding bars 21, these dams 23 are positioned such that the desired cutting angle is achieved between the dams 23 of the stator 17 and the grinding bars 21 of the rotor 16. The relative movement between the rotor 16 and the stator 17 causes the cutting angle between the grinding bars 21 of the rotor 16 and the dams 23 of the stator 17 to close in the direction of the inner diameter. The dams 23 thus also take over the function of the grinding bars of the stator.

[0052] Optimizing Hydraulic Capacity:

[0053] It has been shown that a reduced height of the grinding bars 21 due to wear can reverse the flow direction in the stator 17 from backflow to forward flow. The dams 23 at grinding element 2 of the stator 17 prevent this forward flow, and thus a reduction in hydraulic capacity can occur towards the end of the service life. For this reason, the inner area of ​​the stator 17 is not equipped with dams 23 in treatment elements used for organic chlorinated filtration (OCC). This allows suspension to flow in the conveying direction and contribute to the capacity, especially in cases of advanced wear of the grinding bars 21.

[0054] To improve the hydraulic capacity of grinding surfaces consisting of grinding elements with a small cutting angle, e.g., 30–40°, the angle distribution of the grinding bars 21 from rotor 16 to stator 17 is modified. On stator 17, the grinding bars are arranged radially aligned at 0° with a deviation of + / -5°. The grinding bars 23 of rotor 16 are arranged with a predetermined radial inclination angle, for example, 30° to the radial. This improves the pumping action of the grinding surfaces of rotor 16 and stator 17.

[0055] By aligning the grinding bars 21 at 0° on the stator 17, a casting pattern can be eliminated. This solution of a different grinding bar angle between rotor and stator is suitable for assemblies where the designs of rotor 16 and stator 17 differ.

[0056] Of the 35 openings, the laterally extending grinding bars 21 are designed with a greater thickness. This reduces damage caused by foreign parts / contaminants in the suspension.

[0057] Figure 6 shows grinding bars 21. The grinding bars 21 have an axially extended extent 31, also referred to as the grinding bar height. The volume of the grooves 22 bounded between the grinding bars is thereby increased in this region. The grinding element 2 has a constant axial extent. The gap width remains unchanged. The base plate 18 supporting the grinding bars has a reduced thickness 19 in the radially inner region. The reduced thickness of the base plate 18 compensates for the greater extent of the grinding bars 21 in the inner region. This results in improved suspension flow in the radially inner region.

[0058] The hydraulic capacity of the grinding segments typically decreases with increasing wear, and premature removal of the grinding elements 2 may be necessary due to insufficient hydraulic capacity. To increase the hydraulic capacity, for example, with a remaining grinding bar height of 2 mm in the radially outer third of the grinding element 2, the open area in the inner region of the grinding element 2 can be significantly increased by increasing the height of the grinding bars 21 radially inwards, for example by 1 mm to a maximum of 3 mm, and in particular by 1.5 mm. The heights of the grinding bars 21 in the radially outer region are generally initially 10 mm, 8 mm, or 6 mm. With a continuous increase of 1.5 mm radially inwards, a grinding bar height of 8 mm at the outer radius would thus initially result in a grinding bar height of 9.5 mm.

[0059] Figure 7 shows the path of the treatment elements 20 of the rotor and stator assembly. The path of the grinding bars of the rotor 16 is shown in the foreground. In the background, the path of the grinding bars 21 and the dams 23 of the stator 17 is shown. The treatment elements facing each other in the gap and their interaction result from the relative position of the treatment elements to one another. The treatment edge 25 is clearly visible.

[0060] Reference symbol list

[0061] 1 Grinding arrangement

[0062] 2 grinding element

[0063] 3 grinding gap

[0064] 4 grinding surfaces

[0065] 5 Inlet

[0066] 6 Procedure

[0067] 8 grinding segment

[0068] 9 Axial extension of grinding element

[0069] 10 Rotation axis

[0070] 16 Rotor

[0071] 17 Stator

[0072] 18 Base plate

[0073] 19 Axially reduced thickness of the base plate

[0074] 20 treatment elements (walkways, dams)

[0075] 21 grinding bars

[0076] 22 Nut

[0077] 23 Dam

[0078] 24 angle grinding bar - dam

[0079] 25 Treatment edge

[0080] Section 26

[0081] 27 division

[0082] 28 Thickened area of ​​the grinding bar

[0083] 29 Area with tapered grinding bar

[0084] 30 strength grinding bar

[0085] 31 Axial extended extent of grinding bars

[0086] 32 Obtuse angle

[0087] 35 Breakthrough

[0088] S Fiber suspension

Claims

Patent claims 1. Grinding element (2) for a refiner with grinding bars (21) and grooves (22) extending between the grinding bars (21) and wherein some of the grinding bars (21) divide radially outwards in a division section (26), characterized in that the grinding bars (21) after the division (27) in the division area (26) have a lesser thickness in the circumferential direction in at least one section than outside the division section (26).

2. Grinding element (2) according to claim 1 , characterized in that the grinding bars (21 ) after the division (27) have a thickness that is at least 15% lower, preferably at least 30% lower.

3. Grinding element (2) with openings for fastening according to one of the preceding claims, characterized in that grinding bars (21) adjacent to the openings (35) in the circumferential direction are thickened.

4. Grinding element (2) according to claim 3, characterized in that the thickened grinding bars (21) are part of a dividing section (26) before the dividing (27).

5. Grinding element (2) according to one of the preceding claims, characterized in that connecting dams (23) are formed between grinding bars (21) adjacent in the circumferential direction.

6. Grinding element (2) according to one of the preceding claims, characterized in that the dams (23) are formed only in the radially outer region.

7. Grinding element (2) according to one of the preceding claims, characterized in that the grinding bars (21 ) are arranged in a purely radial manner outside the division section (26).

8. Grinding element (2) according to one of the preceding claims, characterized in that the grinding bars (21 ) in the dividing section (26) after the dividing section (27) have a smaller thickness in the circumferential direction over at least 1 / 3, preferably 2 / 3, of the length of the section after the dividing section (27) in the dividing section (26).

9. Grinding element (2) according to claim 8, characterized in that the width of the grooves (22) and the thickness of the grinding bars are the same in the circumferential direction outside the division sections (26).

10. Grinding element (2) according to one of the preceding claims, characterized in that the grooves (22) and / or the grinding bars (21) in the division section after the division (27) have a reduced extent in the circumferential direction.

11. Grinding element (2) according to one of the preceding claims, characterized in that dams (23) are provided in the grooves (22), preferably in the radially outer region of 2 / 3 of the radial extent of the grinding element (2).

12. Grinding element (2) according to claim 11 , characterized in that an obtuse angle (32) to the radial inside is provided between dam (23) and grinding bar (21 ) at the radially inner connection point.

13. Grinding element (2) according to one of the preceding claims, characterized in that the grinding bars (21 ) are formed radially inside with an axial extent (31 ) that is at least 15%, preferably 25% larger than radially outside.

14. Grinding arrangement (1) for grinding aqueous suspended cellulose fibers between two grinding surfaces (4) forming a grinding gap (3) and rotating relative to each other, wherein at least one of the grinding surfaces (4) comprises grinding elements (2) according to claim 11, wherein characterized in that the dams with the grinding bars have an obtuse angle radially inwards to form a treatment edge (25) on the dam (23) for the flow of suspension by the relative movement of the grinding surfaces.

15. Grinding arrangement (1) for grinding aqueous suspensions cellulose fibers between two grinding surfaces (4) forming a grinding gap (3) and rotating relative to each other, wherein at least one, preferably both of the grinding surfaces (4) have grinding elements (2) according to one of the preceding claims.

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